Hansen, J., Mki. Sato, P. Kharecha, G. Russell, D.W. Lea, and M. Siddall, 2007: Climate change and trace gases. Phil. Trans. Royal. Soc. A, 365, 1925-1954, doi:10.1098/rsta.2007.2052. [PDF]

Paleoclimate data show that the Earth’s climate is remarkably sensitive to global forcings. Positive feedbacks predominate. This allows the entire planet to be whipsawed between climate states. One feedback, the “albedo flip” property of water substance, provides a powerful trigger mechanism. A climate forcing that “flips” the albedo of a sufficient portion of an ice sheet can spark a cataclysm. Ice sheet and ocean inertia provides only moderate delay to ice sheet disintegration and a burst of added global warming. Recent greenhouse gas (GHG) emissions place the Earth perilously close to dramatic climate change that could run out of our control, with great dangers for humans and other creatures. Carbon dioxide (CO2) is the largest human-made climate forcing, but other trace constituents are important. Only intense simultaneous efforts to slow CO2 emissions and reduce non-CO2 forcings can keep climate within or near the range of the past million years. The most important of the non-CO2 forcings is methane (CH4), as it causes the 2nd largest human-made GHG climate forcing and is the principal cause of increased tropospheric ozone (O3), which is the 3rd largest GHG forcing. Nitrous oxide (N2O) should also be a focus of climate mitigation efforts. Black carbon (”black soot”) has a high global warming potential (~2000, 500, and 200 for 20, 100 and 500 years, respectively) and deserves greater attention. Some forcings are especially effective at high latitudes, so concerted efforts to reduce their emissions could still “save the Arctic”, while also having major benefits for human health, agricultural productivity, and the global environment.

Several figures, captions, and text have been reproduced in this post under fair use. No copyright claims are made and readers are encouraged to read the entire paper, freely available at the link at the top of this post.

Much has been made about the lag between temperature and greenhouse gas concentrations. In the past, I’ve refered to a post at Real Climate. In this post, I’ll just quote Hansen et al.

Figure 1a reveals remarkable correspondence of Vostok temperature and GHG [greenhouse gas] climate forcing. The temperature change appears to usually lead the gas changes by typically several hundred years, as discussed below and indicated in figure 1b. This suggests that warming climate causes a net release of these GHGs by the ocean, soils and biosphere. GHGs are thus a powerful amplifier of climate change, comparable to the surface albedo feedback, as quantified below. The GHGs, because they change almost simultaneously with the climate, are a major ’cause’ of glacial-to-interglacial climate change, as shown below, even if, as seems likely, they slightly lag the climate change and thus are not the initial instigator of change.

The temperature-GHG lag is imprecise because the time required for snow to pile high enough (approx. 100m) to seal off air bubbles is typically a few thousand years in central Antarctica. The estimated age difference between ice and its air bubbles is accounted for in the time-scale of figure 1, which refers to the ice age. Despite multiple careful studies, uncertainties in the ice-gas age differences for the Vostok ice core remain of the order of 1 kyr [1000 years] (Bender et al. 2006). Therefore, we can only say with certainty that the temperature and gas changes are nearly synchronous. Data from a different Antarctic (Done C) ice core with slightly higher snow accumulation rate (Monnin et al. 2001) and an independent analysis based on argon isotopes (Caillon et al. 2003) support temperature leading GHGs by ca 600-800 years. In addition, carbon cycle models yield increases of GHGs in response to warming oceans and receding ice sheets. Ice cores from Maud Land (EPICA 2006), which has very high snow deposition rates, should establish leads and lags accurately, but the present paper has only slight dependence on that result.

To summarize, temperature did lead greenhouse gas concentrations in the ice core data. However, once the temperatures increased, greenhouse gases accumulated in the atmosphere and caused additional warming. This is a positive feedback, where an initial positive perturbation in temperature causes a much larger, and continually cycling, increase in temperature.

This figure shows Hansen’s estimated climate forcings from 1750-2000. It is interesting to see how these compare to the latest IPCC report, which is the estimated forcing for 2005.

As can be seen, the estimated forcing due to CO2 has increased from the 1750-2000 estimate to the 2005 estimate. However, during that same time, the estimated radiative forcing from the combined CH4 and O3 (tropospheric) has decreased. Forcing due to CFCs has also decreased. Land use and aerosols (combined direct and indirect) estimates have remained roughly the same. And the radiative forcing from the sun has decreased by over a factor of 2.

Does this mean that either Hansen or the IPCC must be wrong? No, it just means that the forcings change over time. In the 1700s, there was less CO2 in the atmosphere, so the radiative effects were lessened. Hansen includes the forcing effects of H2O (not feedbacks) in his second column. The IPCC does not have any H2O forcings in their figure (SPM-2). The forcing effects of H2O appear to be small from Hansen’s figure, but it could explain the difference between the two numbers included in the “CH4 / O3″ row above - which includes only CH4 and O3 from the IPCC. The decrease in radiative forcing from CFCs is expected. With the decreased production of CFCs since the signing of the Montreal Protocol, there is obviously less CFCs in the atmosphere to cause any radiative effects. Because of the long residence time of CFCs in the stratosphere (50-100 years), the negative effects are still being felt. In fact, atmospheric CFC concentrations have decreased only ~7% since the passage of the Montreal Protocol.

This figure has been altered in appearance, but no substantive changes were made. We can see in panel A (observations), that overall there is a large warming trend. Most of this warming is in the high Northern lattitudes - Canada, Russia, Arctic, and to a lesser extent the southern high lattitudes - South Africa, Australia.

Panel D is especially interesting. It shows the temperature changes due only to the black carbon on snow albedo effect. Black carbon is obviously black. Snow is obviously white, at least in the visible part of the spectrum. As we know, black objects tend to absorb more radiation than white objects, and are thus heated more than white objects. When black carbon falls on snow, it absorbs the incident radiation and heats the objects around it - the snow. This in turn melts the snow, but the black carbon doesn’t melt. Some may get washed away by the melting snow, but some will stay on top of the newly exposed snow. The carbon continues to heat the snow. This effect can only be experienced where there is snow; that means that the black carbon albedo effect only affects the high lattitudes. There are actually some areas of cooling due to this feedback. One of note is along Western Australia. As far as I know, this region does not have extensive snow cover, so there should be no black carbon feedback. Also, we see an effect over the oceans. Now I know that over the Arctic ocean there is an ice cap and both polar regions have ice bergs, but I don’t think there is any sea ice off the coast of California and streaching to Hawaii.

(No Ratings Yet)

 Loading ...

Related Posts:

Trackback URI |